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In an environment where all plants are resistant to certain parasites, a rare breed which has a mechanism against this resistance has free play - lots of food and no competition. However, in plants which do not have the resistance, this rare parasite breed may be at a disadvantage compared to parasites who do not have the mechanism against that resistance (even if only because they do not waste energy on a defensive mechanism they don't need).

By mixing resistant and unresistant plants, it is probably possible to maintain high enough a competition of parasites without that defensive mechanism to prevent the ones with the mechanism from developing. It's natural selection at work :)